Source-coupled differential low-swing driver circuits

Abstract

A novel source-coupled differential driver circuit fully compatible with digital visual interface TMDS signaling specification is disclosed. Driven output signals are connected to the source terminals of driving switches in the invention circuit, minimizing the detrimental impact of miller coupling capacitance between gate nodes and driven output nodes upon output slew-rate, enabling higher frequencies of operation. Output signal undershoots and overshoots are also mitigated by the invention driver circuit due in part to the in-phase relationship of the gate node to the driven output node of a driver switch. Overall link performance is significantly enhanced by this SCDL driver architecture due to improved slew rates and signal integrity.

Description

TECHNICAL FIELD OF THE INVENTION[0001]Embodiments of the invention relate to electronic circuitry commonly employed to transmit data and clock in the form of binary signals over lengths of interconnect to other electronic circuits, devices and systems. Such circuitry falls under the category of Data Communication Circuits.BACKGROUND & PRIOR ART[0002]Low Voltage Differential Signaling (LVDS) is ubiquitous in the art. The popularity of this signaling technique arose in part from the expectation of substantially reduced power consumption because of the low (˜350 mV) swing on the lines as well as the differential nature of the signals that enabled accurate recognition despite static or dynamic variations in ground or supply voltages between the transmitting and receiving systems. Low signal swing also permits faster signal transitions, enabling higher rates of data transmission. Additionally, the differential and low-swing nature of signals also minimizes electromagnetic interference (E...

AI Technical Summary

Benefits of technology

[0006]The invention improves upon prior art substantially through a novel source-coupled differential low-swing (SCDL) driver circuit. Output signals are connected to the source terminals of driving switches in the invention circuit, minimizing the detrimental impact of miller coupling capacitance between gate nodes and driven output nodes upon output slew-rate, enabling higher frequencies of operation. Output signal undershoots and overshoots are also mitigated by the invention driver circuit due in part to the in-phase relationship of the gate node to the driven output node of a driver switch. A cascoded current source additionally ensures very high output impedance and mitigates current modulation. Due in part to the controlled output slew, the SCDL driver lends itself to transmitter emphasis, extending the reach of DVI links.

Problems solved by technology

These are undesirable characteristics of DVI electrical and physical layers and TMDS that necessitate high quality and associated cost in video cable construction in order to maintain acceptable bit error rates.

Prior art TMDS CMOS drivers as illustrated in FIG. 1 have significant functional disadvantages that impact output signal integrity.

This leads to a slower overall ‘slew-rate’ or rate of change of the output, limiting high-speed performance.

Additionally, the miller cap charge and discharge in combination with device packaging parasitic elements leads to overshoots and undershoots in the differential signal developed in the output signal pair.

Also, the transition of the input differential signals to the current-steering switches connected at their sources to the constant current-source modulates this current to an extent, further impacting signal integrity.

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